1. Field of the Invention
The present invention relates to a structure for securing the electrical safety of a centrifugal separator.
2. Description of the Related Art
The centrifugal separator is configured so that a rotor caused through a tube and a bottle to hold a sample to be separated is accommodated in a rotor chamber (rotating chamber), and that the rotor is rotated at high speed by a drive unit, such as a motor, in a case where an opening portion of the rotor chamber is hermetically closed by a door, to thereby separate and purify the sample held by the rotor. The rotational speed of the rotor varies with the use thereof. Generally, there are provided families of products having rotational speeds that widely range from a relatively low speed, the maximum value of which is about several thousands revolutions per minute (rpm), to a high speed, the maximum value of which is about 150,000 rpm.
FIG. 6 shows the configuration of a known centrifugal separator. The centrifugal separator comprises a motor housing (casing) 5 of a motor 5a serving as a rotary drive source, a rotating spindle (a shaft) 5b rotatably connected to the motor 5a, a rotor 1 fixed to the rotating shaft 5b and adapted to hold a sample to be separated, a rotor chamber 2 adapted to accommodate the rotor 1 and to have an opening portion 2a in the top surface thereof, a door 3 openably/closeably provided in the opening portion 2a of the rotor chamber 2, a door lock mechanism 4 adapted to restrict the opening/closing of the door 3, a door opening/closing detector 12 adapted to detect the opening/closing of the door 3, and a control unit 60 adapted to control the motor 5a and the door lock mechanism 4. The motor housing 5, the rotor chamber 2, the door lock mechanism 4, and the control unit 60 are accommodated in a casing (frame) 10.
When the door 3 of the rotor chamber 2 is opened in such a centrifugal separator, a user may touch the rotating shaft 5b of the motor 5a and the rotor 1 that may electrically be conducted to the rotating shaft 5b. Thus, generally, an electrically insulating layer is provided between the winding of the motor 5a and the rotating shaft 5b to thereby prevent a user from getting an electrical shock.
Further, to prevent a power supply voltage from being generated on the rotating shaft 5b even when such an insulating layer may cause dielectric breakdown, an electric shock guard means is doubled by electrically grounding the motor housing 5 of the motor 5a through a ground connection wire 8. Usually, the casing 10 of the centrifugal separator is connected to a ground connection wire 9. Thus, the ground connection wire 8 of the motor housing 5 is electrically connected to a part of the casing 10 of the centrifugal separator placed in the vicinity of the motor housing 5. Incidentally, the value of a leakage current of the centrifugal separator is limited to a value, which is predetermined according to JIS (Japanese Industrial Standards) safety standard or to IEC (International Electrotechnical Commission) safety standard not to seriously affect a human body, or less (for instance, 3.5 mA or less) Also, it is required to place a plurality of electric shock guard means at members of a centrifugal separator, which have possibilities of being touched by users. Also, it is prescribed that in a case where an insulation part is used as the electric shock guard means, the insulation part should have a high withstand voltage (for instance, 1300V or higher).
However, in some condition in which the centrifugal separator is used, a user may use the centrifugal separator in an environment in which no grounding equipment is provided. In this case, the aforementioned electric shock guard means utilizing the grounding cannot be employed. Thus, an insulation transformer 13 is used as another ordinary electric shock guard means, as illustrated in FIG. 5, to thereby ensure safety. Also, sometimes, a method of performing double insulation or reinforced insulation on the casing of the motor itself by using an insulating layer to insulate the winding of a motor is performed as a still another means for assuring safety.
Incidentally, regarding the related art, JP-UM-B-60-20753 discloses the technique of preventing occurrence of an electric shock by providing an electrical insulating layer between the rotor winding and the rotating shaft of a motor, or what is called a double insulation technique of constructing also a motor casing by an insulating material. Further, JP-A-9-187428 discloses the technique of preventing the generation of a leakage current by using the insulation transformer. Furthermore, JP-A-2001-87677 discloses the technique of constituting a centrifugal separation rotor attached to the rotating shaft of a motor by an insulating material to thereby ensure safety in a case where a user touches the rotor and so on.
However, the structure using the aforementioned insulation transformer 13, and the structure of the motor 5a, to which the double insulation or the reinforced insulation is applied, are advantageous in a case where the ground connection wire 9 is not ground-connected, such structures have a problem that the structures cause an increase in the cost of the centrifugal separator. Further, the technique described in the aforementioned JP-A-2001-87677 is subjected to a constraint that the process material of the rotor is an insulating material. Thus, it is difficult to combine a rotor, which is made of a generally used metallic material, with a centrifugal separator body.
Furthermore, when the aforementioned insulation transformer is used or when the double insulation or the reinforced insulation of the motor is performed thereon, not only the employment of a plurality of electric shock guards but that of countermeasures to limit the value of a leakage current (an electric current flowing through the body of a user when the user touches the rotor) generated through floating capacity (stray capacitance) to a leakage current value, which is determined according to the safety standards, or less is performed.